1. Field of the Invention
The present invention relates to power converters and more particularly, high voltage, large capacity power converters which connect in series multiple self-turn-off-semiconductor devices.
2. Description of the Related Art
In recent years, power converters which can supply high voltages and large currents have been desired. In particular, the ability to supply 2000.about.3000 A or more at 100.about.500 KV is required by DC transmission and the like.
FIG. 1 is a block diagram of the overall composition of a prior art power converter and of a switch unit. FIG. 2 is a block diagram of the control signal supply circuits for the gate circuits which compose the power converter in FIG. 1.
In FIG. 1, AC power source 1 is connected to converter 3 via transformer 2.
This converter 3 provides self-turn-off-semiconductor devices which are bridge-connected. The following is the description of the case when the composition uses gate turn-off thyristors (hereafter, referred to simply as "GTO") as one example. The DC current which is the output of this converter is smoothed via DC reactor 4 and supplied to DC circuit (load) 5.
The parts to which symbols 61 and 71 are assigned are GTOs. The parts to which symbols 81 and 91 are assigned are diodes. When GTOs 61 and 71 turn off, the current which was flowing in GTO 61 and 71 is led to capacitor 101. Capacitor 101 operates as a so-called snubber capacitor which restricts the forward voltage rising rates dv/dt of GTOs 61 and 71 to the permitted values for the GTO elements or less when GTOs 61 and 71 turn off. The charge accumulated in capacitor 101 is regenerated to AC power source 1 or DC circuit 5 via converter 3 when GTOs 61 and 71 turn on. The above GTOs 61 and 71, diodes 81 and 91 and capacitor 101 compose U-phase switch unit 111.
U-phase switch units 112.about.11n are composed in the same way.
Also, V-phase switch units 121.about.12n, W-phase switch units 131.about.13n, X-phase switch units 141.about.14n, Y-phase switch units 151.about.15n and Z-phase switch units 161.about.16n are composed in the same way.
Converter 3 is composed by bridge-connecting arms 11.about.16 which are formed from the above U-phase switch units.about.Z-phase switch units.
In FIG. 2, the composition of the arms is illustrated, taking U-phase arm 11 as an example.
The parts designated by symbols 191.about.19n and 201.about.20n are gate circuits which output gate signals 171.about.17n and 181.about.18n in order to turn on or turn off the respective GTOs 61.about.6n and 71.about.7n.
The control signals which control the turning on and off of each gate circuit 191.about.19n and 201.about.20n are transmitted from control system 25 via control signal lines 231.about.23n and 241.about.24n. Generally, optical cables are used for control signal lines 231.about.23n and 241.about.24n, and optical insulation is performed between control system 25 and gate circuits 191.about.19n and 201.about.20n.
The power for driving gate circuits 191.about.19n and 201.about.20n is obtained from power supply unit 27. Power supply unit 27 is connected to AC power source 26 which is obtained externally such as utility lines, and outputs a stabilised AC voltage with little voltage fluctuation. Insulation between power supply unit 27 and gate circuits 191.about.19n and 201.about.20n is performed by transformers 211.about.21n and 221.about.22n.
V-phase arm 12, W-phase arm 13, X-phase arm 14, Y-phase arm 15 and Z-phase arm 16 are composed in the same way.
The following is a description of the operation of a prior art power converter, referring to FIG. 1 and FIG. 2.
For example, in the state in which U-phase switch unit 111 is turned on, arm current I branches into a current flowing in the path of GTO 61 and diode 81 and a current flowing in the DC circuit of GTO 71 and diode 91 inside the U-phase switch unit.
When GTOs 61 and 71 turn off, arm current I shifts the conducting path to diode 81, capacitor 101 and diode 91. When capacitor 101 is charged, U-phase switch unit 111 becomes off by diodes 81 and 91 turning off.
When GTOs 61 and 71 turn on once again, capacitor 101 discharges the accumulated charge to AC power source 1 or DC circuit 5 via GTOs 61 and 71. After the discharge of capacitor 101, diodes 81 and 91 turn on, and arm current I branches as described above.
U-phase switch units 112.about.11n also operate in the same way.
The above operation is also performed by each of the other switch units 121.about.12n, 131.about.13n, 141.about.14n, 151.about.15n and 161.about.16n.
However, with the above-mentioned power converter, when used as a high voltage, large capacity power converter, as shown in FIG. 1 and FIG. 2, U-phase arm 11 is composed by connecting in series a large number of switch units 111.about.11n. In consequence of this, gate circuits 191.about.19n and 201.about.20n are required for respective switch units 111.about.11n.
As described above, the insulation of gate circuits 191.about.19n, and 201.about.20n and power supply unit 27 is performed by transformers 211.about.21n. However when, for example, U-phase arm 11 becomes high-voltage, it is sometimes difficult to keep insulation by transformer because the voltages to earth in the primary sides and secondary sides of transformers 211.about.21n differ greatly.
In this case, power cannot be supplied for driving gate circuits 191.about.19n and 201.about.20n. Therefore, this becomes an impediment to producing a high voltage, large capacity power converter.